FDOT has a cool new website with high resolution pictures of just about any highway-related subject you can think of.
http://www.seefloridago.com/
Do any other DOT's have something similar?
MODOT and WSDOT are active on flickr.
http://www.flickr.com/photos/wsdot/ (http://www.flickr.com/photos/wsdot/)
http://www.flickr.com/photos/modot/ (http://www.flickr.com/photos/modot/)
Maybe some others are on Flickr as well.
edit:
http://www.flickr.com/photos/delawaredot (http://www.flickr.com/photos/delawaredot)
http://www.youtube.com/michigandot (http://www.youtube.com/michigandot)
http://www.youtube.com/user/TxDOTpio (http://www.youtube.com/user/TxDOTpio)
http://www.youtube.com/user/TDOTnews (http://www.youtube.com/user/TDOTnews)
ConnDOT has a Photolog site with streaming video of the official photolog of most major routes in the state of CT, which is pretty cool.
Most videos have a jump-to-reference section below the video window.
(hint: i watch them on fast forward, it's a little closer to realtime until i get to what I'm looking for)
http://www.ct.gov/dot/cwp/view.asp?a=1617&q=439946
Washington has a pretty slick interface as well, now with high-res photos
http://www.srview.wsdot.wa.gov/
ODOT has a less friendly videolog, but it's still there
http://www.oregon.gov/ODOT/TD/TDATA/rics/PublicRoadsInventory.shtml#Digital_Video_Log (scroll down and click External)
Caltrans also has a web page (http://video.dot.ca.gov) dedicated to live streaming and still-image traffic cams. Most of the links on that page are live streaming cams but the extreme northern region and central valley region (Kern, Tulare, Fresno and Madera counties) have still-image cams. I visit this page a lot to see conditions on I-80 and US 50 in the Sierra Nevada mountains.
Caltrans District 4 which covers the San Francisco Bay Area has it's own photo gallery (http://www.dot.ca.gov/dist4/photography) which contains photos taken at many of the construction sites in the region.
Ohio has a nice one....organized by topic as well as county, it can be a bit cumbersome to wade through, if you are patient, you will find a LOT of good piccys of early Ohio route markers and other signs from the 20s to the early 1960s
http://www.odotonline.org/photoarchive/
Arkansas:
http://www.arkansashighways.com/Photo/AHTDPhoto.aspx (http://www.arkansashighways.com/Photo/AHTDPhoto.aspx)
PennDOT VidLog (http://www.dot7.state.pa.us/ividlog/video_locate.asp)
Alabama also has a live stream of its cameras too.
http://www.dot.state.al.us/internetdocs/apps/bureaus/design/its/publicits/CameraList.aspx (http://www.dot.state.al.us/internetdocs/apps/bureaus/design/its/publicits/CameraList.aspx)
NYSDOT has a Photolog, but to my knowledge it is not online. However, if you're visiting Albany, NY, you can make an appointment to check out the photolog.
https://www.nysdot.gov/divisions/engineering/technical-services/highway-data-services/photolog-unit (https://www.nysdot.gov/divisions/engineering/technical-services/highway-data-services/photolog-unit)
VDOT's official Flickr (http://www.flickr.com/photos/vadot/)
As far as I know the MTO doesn't post pictures in a photo gallery, I have only found pictures in their newsletters and construction reports